1. Field of the Invention
The present invention relates to a driver circuit for driving a charge pump circuit.
2. Description of the Related Art
Recent information terminals such as a cellular phone and a PDA (Personal Digital Assistance) are provided with light emitting diodes (hereinafter, referred to as LEDs) employed as, for example, a back light of a liquid crystal display. In such information terminals, a Li-ion battery is often employed, but the output voltage thereof is normally about 3.5 V and about 4.2 V at full charge. However, since a voltage of 4 V or higher is required to drive an LED, the battery voltage must be stepped up by use of a power unit such as a switching regulator in accordance with need to thereby supply the stepped up voltage to the LED.
A charge pump circuit capable of switching a step-up ratio as disclosed in Japanese Patent Laid-Open Publication No. Hei 6-78527 may be employed as a power unit for driving an LED. In this case, the step-up ratio is set to 1 if the voltage of a battery is sufficiently high, and is increased as the battery voltage decreases, thereby allowing the LED to be stably driven.
When the step-up ratio is set higher than 1 to perform a step-up action, a switching transistor must be switched on-off at a frequency of about 1 MHz for controlling charging and discharging of a capacitor of the charge pump circuit. A control circuit for the charge pump circuit has a built-in oscillator for this purpose. However, when the step-up ratio of the charge pump is set to 1, the step-up action is not performed. In this case, it is sufficient to bypass the input and output connectors of the charge pump circuit, and the switching transistor is not required to be switched on-off. Thus, if the 1 MHz oscillator is continuously operated, electric power is consumed unnecessarily.